Variable speed HVAC with two zones.

[sjn37]

I have a two story house with two zones (upstairs/downstairs). Last year I replaced my old AC with a new variable speed system from Lennox (SL280UH090V60C). Recently I have started noticing that the system does not behave the way I would expect the system to behave, so I started to collect data and make some graph. I expected a variable speed system to run for long periods at a time at very low speeds. What I see is that the system turns on/off much more frequently and run mostly at full speed. I have plotted the demand for both zones below. This graph leads me to several questions.

1. It always starts at 62% and never runs slower than that. It used to start at 42%, but a technician made some adjustment and now it is 62%. Why does it never run slower?

2. After running for a while and reaching the target (I assume) it turns off completely. I had expected it to just go down to a slow speed to maintain the temperature.

3. If both zones are calling at the same time, the demand from both zones is 100%. Is it not possible for the system to run at lower speeds with both zones open?

I have had several technicians look at it, but they don't seem to even understand what the issue is. The system is maintaining the set temperature, both upstairs and downstairs, so there is no problem. Last technician also called Lennox support and walked through some diagnostics steps, but they concluded the system was working as expected. Everything I have read about variable speed systems says that they run slower for longer time and fewer on/off cycles. Last week when it was in the 90's here my system turned on/off ever eight minutes. Now it is a little cooler and it does nor toggle as often.

I was hoping some expert here would have some insight. I feel like a crazy person when no one ever understand why I think there is an issue.

[pecmsg]

This is why I preach
2 stories
2 systems.

Someone has to be there taking measurements and getting data to determine what’s going on.

[sjn37]

I understand that two systems would have been better, but unfortunately, the house was build with a single system, so zoning is the only (reasonable) option. But you agree that this is not the expected behavior for a variable speed system?

[pecmsg]

I understand that two systems would have been better, but unfortunately, the house was build with a single system, so zoning is the only (reasonable) option. But you agree that this is not the expected behavior for a variable speed system?

I’m not
I can’t say

Doesn’t sound right though!

[BaldLoonie]

What's the zone panel? If not a matched Lennox system, may not work nearly as well.

[sjn37]

What's the zone panel? If not a matched Lennox system, may not work nearly as well.

It is an iHarmony zone controller.

[ss120396]

Sounds like normal operation to me. The iHarmony system doesn't modulate individual zones. Each zone has a fixed airflow setpoint, and it will modulate the system to meet that setpoint and stay there, when a single zone is calling. When multiple zones are calling it will modulate the system to meet the sum of the airflow setpoints.

The airflow setpoints for each zone need to be set based on the Manual J load calculation for that zone as well as what the ductwork can handle. If this wasn't done properly, there may be room to turn the zones down so your system modulates lower.

[sjn37]

Sounds like normal operation to me. The iHarmony system doesn't modulate individual zones. Each zone has a fixed airflow setpoint, and it will modulate the system to meet that setpoint and stay there, when a single zone is calling. When multiple zones are calling it will modulate the system to meet the sum of the airflow setpoints.

The airflow setpoints for each zone need to be set based on the Manual J load calculation for that zone as well as what the ductwork can handle. If this wasn't done properly, there may be room to turn the zones down so your system modulates lower.

Thanks for that insightful response. This is the most useful information I have gotten so far from any one. Do you happen to know where I can find the manual so I can show this to my installer?

Why does the demand ramp up slightly when a single zone is calling? I would expect it to go straight to the set point value? Also, in my picture it always starts at 42%, but after they adjusted (reduced the max) airflow settings, it now always starts at 62% instead.

Are there any other zone controllers that do modulate airflow for each zone? When the sales representative explained the benefits of a variable speed system, he said it would run longer time with lower speeds. This does not seem to be the case here, and you are saying that would not happen with a zoned system?

[ss120396]

To be honest, I'm not a Lennox guy or a super expert on this system, I just know the basics from having run into it a couple times. If I recall, the manual isn't explicit about the fact that the zones don't modulate, but I quickly figured it out from seeing the way the system operated, the way the zone dampers operated, and the fact that the settings screens only had single airflow setpoints for the zones.

I'm not sure why the demand ramps up with one zone. You really need a tech who both understands the fundamentals of ductwork and airflow, and has been trained specifically on this zone system.

Personally I'm more familiar with the Trane and Carrier communicating zone systems, which do modulate the individual zone dampers, and thus, the system power, even when a single zone is calling. I think this is a more sophisticated control scheme that takes better advantage of the variable speed equipment. Some techs prefer the Lennox way of doing things though, because the airflow to each zone is constant and you don't need to worry about things like the performance of registers declining when airflow is turned down.

Your variable speed system is turning down when there is less demand - ie. when only one zone is calling. It just isn't modulating the individual zones. If you turn off zoning, it should start modulating the system based on difference from the setpoint, rather than the number of zones calling.

[sjn37]

To be honest, I'm not a Lennox guy or a super expert on this system, I just know the basics from having run into it a couple times. If I recall, the manual isn't explicit about the fact that the zones don't modulate, but I quickly figured it out from seeing the way the system operated, the way the zone dampers operated, and the fact that the settings screens only had single airflow setpoints for the zones.

I'm not sure why the demand ramps up with one zone. You really need a tech who both understands the fundamentals of ductwork and airflow, and has been trained specifically on this zone system.

Personally I'm more familiar with the Trane and Carrier communicating zone systems, which do modulate the individual zone dampers, and thus, the system power, even when a single zone is calling. I think this is a more sophisticated control scheme that takes better advantage of the variable speed equipment. Some techs prefer the Lennox way of doing things though, because the airflow to each zone is constant and you don't need to worry about things like the performance of registers declining when airflow is turned down.

Your variable speed system is turning down when there is less demand - ie. when only one zone is calling. It just isn't modulating the individual zones. If you turn off zoning, it should start modulating the system based on difference from the setpoint, rather than the number of zones calling.

Thanks again, very useful information. The problem is that I don't know how to find someone who understands the system that well. The technicians that have been out here don't seem to understand what the problem is. One even called Lennox support to go through diagnostics and they said everything was fine. I guess if you don't have experience with other systems you don't know the difference.

I am mostly annoyed that this was explained wrong to me when I purchased the system. I was told it would modulate even with two zones. Had I known maybe I would have chosen a different system.

[pecmsg]

Calling tech support is needed with today equipment. Not understanding what there saying is not acceptable.
Tell the contractor you want a more experienced tech.